1. Field of the Invention
With reference to the classification of art as established in and by the U.S. Patent Office the present invention pertains to the general class entitled, "Electric Heating" (Class 219) and more particularly to the subclass therein entitled, "With heater-unit housing, casing or support means" (Subclass 520) and subclasses thereunder.
2. Description of the Prior Art
Exposed heating elements using electrical energy have been known for many years. When and where radiant heating is used in commercial manufacturing steps with high temperatures, the resistance unit is conventionally supported by an insulation board or backing disposed to retain their properties at high temperatures such as thirty-two hundred degrees Fahrenheit. Recent patents have shown ribbon elements which are necessarily thin and delicate in order to provide a quick response time and for economy of material.
Ribbons with convolutions in a corrugated manner have a tendency for buckling or distortion and require supports for holding these ribbon portions to an insulation backing. Punched holes and mounting supports usually cause a reduction of the life expectancy of the ribbon element. A small indention is usually created at the support point because the heater strips expand and contract during use. This indention may and does cause the element to burn out in a relatively reduced period of time.
An infra-red heating panel is shown in U.S. Pat. No. 2,857,499 to FEARN but the continuous wire means of Applicant is not present. U.S. Pat. No. 3,119,924 to KUESER shows a heating means having a round wire heating element but the wire element is in a recess with a metal reflector and has no supported element with a securing means to an insulating panel. U.S. Pat. No. 3,525,850 to HAGER, JR. utilizes the corrugated strip above noted. U.S. Pat. No. 3,757,083 to DIETZ, et al., is like the HAGER reference as it pertains to ribbon heaters. U.S. Pat. No. 3,956,612 shows a modular concept with zone heaters and suggests a corrugated strip be used. U.S. Pat. No. 4,100,395 to BALLARD also depicts a heater with corrugated ribbons. U.S. Pat. No. 4,262,192 to GILER shows a straight wire and a suspension system carrying the wire at a distance substantially above the support insulating block.
The prior art above noted may use plural connections to actuate each heater strip. These connections may produce failure and cause excessive voltage consumption. Flat ribbons in corrugated shapes create a large target area and as a consequence residue from the product being cured or processed may drop on this strip and smolder or flame. The presence of this residue often results in deterioration or acidification of the element resulting in element failure (burn-out). Mounting pins used with ribbon elements tend to draw heat away from the point where they engage and support the element. Heating and cooling of the ribbons causes the mounting pins to work in the holes and insulation board so that elaborate securing of these pins to the back of the supporting board requires a large cavity for the ends of the pins and connecting terminals. This pin securing means creates uneven tension in the element and uneven heating. When a ribbon is lightly retained the outer face of the heater may utilize a thin ceramic support sheet which reduces the efficiency of the heater. An additional back-up insulation sheet may be required to direct the heat outward to the product although the outer cover is slower to heat and cool.
The use of sequential serpentine forming of wire into a plurality of strips is known but the device herewith shown uses only two connecting positions and these strips are carried in channels so that the strips are isulated from each other and the position of the strips are outside the retaining support so that all radiant heat is outwardly directed so that hot ends tending to cause failure are minimized.